Temperature control system for electric fluid heater

ABSTRACT

An electric fluid heater of the type in which the fluid is heated by passing electricity through it includes an electronic temperature control system responsive to a temperature sensor in the fluid for controlling the flow of electric current to the heating electrodes. The electronic controller includes means to limit the maximum current passed by the heating electrodes. Heat generated is controlled to demand by varying the duty cycle of back-to-back SCR&#39;s in series with one of the electrodes.

This application is a division of a copending application for ELECTRICFLUID HEATER, Ser. No. 504,814 filed Sept. 10, 1974 now U.S. Pat. No.3,983,559. That copending application is, in turn, a division of anapplication filed Aug. 3, 1973, Ser. No. 385,275, which issued on Sept.30, 1975 as U.S. Pat. No. 3,909,588, By John A. Walker and Dimitri S.Dimitri, assigned to Datametrics Corporation.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an electric fluid heater and, moreparticularly, to a demand-type electric heater in which the fluid isheated on demand only and the output temperature is maintained at acontinuous temperature over wide ranges of flow rates.

The present invention has wide applicability for home installations aswell as factory, laboratory and portable electric heaters for specialpurpose applications. The invention will be described in connection witha water heater having applicability for a home installation since manyof the problems presented by a home hot water heater are solved by theinstant invention. It is expressly understood, however, that theinvention is not limited to the heating of water nor is it limited tohome installation.

2. Description of the Prior Art

Instant electric water heaters have the potential benefit and advantageof eliminating the hot water storage tank usually associated with theconventional gas-fired or oil-fired or electric immersion type hot waterheater systems. In the usual home hot water system a hot water tank ofthe order of 40 or 50 gallon capacity and usually glass lined containsan electric, gas or oil-fired heating mechanism which is preset to keepthe stored water at a predetermined temperature and available for use bythe occupants of the home.

The lowering of temperature of water within the boiler automaticallycauses the firing system to ignite (i.e., to begin applying heat to thewater), thereby supplying more heat to maintain the temperature of thewater within the preset limits. Similarly, as the water is used andfresh cold water is inserted into the tank, the water temperature islowered and again the firing units are ignited for supplying heat to thewater.

The instant hot water heaters do not contain a storage tank, but ratherare connected intermediate the source of cold water and the ultimateuser. A requirement for hot water is made by turning the hot waterfaucet into the "on" position, thereby allowing the cold water to passthrough the heating chamber of the instant hot water system and out thehot water faucet for use by the ultimate user. Electric current iscaused to pass through the water within the heating chamber of theelectric hot water system, thereby instantly heating the water which issupplied to the user as demanded and hence, the term "demand system."

The instant hot water systems should be differentiated from theso-called electric immersion water heaters which normally containresistive elements which are inserted into a water tank or similarcontainer of water where the heat generated by the passage of currentthrough the resistive element is transferred to the water in a heattransfer relationship. These electric immersion water heaters areconventionally used for home hot water heaters and are also used to heatsmall quantities of water, such as in portable containers or individualcup-sized containers for the making of beverages requiring hot water.

This invention is concerned with an instant fluid heating system inwhich the fluid is passed through a chamber containing electrodes andelectric current is passed through the spaced-apart electrodes andthrough the fluid, thereby instantly heating the fluid for use by theultimate consumer.

Instant hot water heaters of the type utilizing immersedcurrent-conducting electrodes are known in the prior art and have beenfully described in the Grupp U.S. Pat. No. 2,529,688 and in theMamoulides U.S. Pat. No. 3,513,281.

The recognized problems associated with instant hot water heatersresults from the fact that the electric current does not flow until thewater is flowing through the heating chamber as evidenced by a demandfor hot water. The rate of heat transfer between the electric currentand the water is therefore very high, and it is imperative thatautomatic temperature control means be provided to regulate the watertemperature during the heating process.

The problem is compounded by the fact that water varies in conductivityfrom location to location as a function of impurities and salts locatedwithin the water. So-called "hard water" contains concentrations ofcalcium, magnesium and iron in varying degrees, resulting in thehardness of the water. The parts per million (p.p.m.) of the dissolvedminerals in the water is a measure of the hard water which is known tovary over a range of 10 p.p.m. to 1400 p.p.m.

In addition, the salinity of the water also varies the conductivity ofthe water, and it is known that the electrical conductivity of the wateris proportional to the percentage of dissolved materials regardless ofwhether the water is of the hard type or soft type. The conductivity ofwater is generally expressed in terms of micro MHos per cubic centimeter(μmho/cm³). Conductivity of municipal water varies enormously. Municipalwater having conductivity as high as 1,660 μmho/cm³ at 25° centigrade isknown in the United States and even higher conductivities may exist.

The wide range of conductivity of the water has severely limited thedevelopment of instant demand electric hot water heater systems sincethe variations of the resistivity of the water of only one-half an ohmin a 220-volt system can vary the demand for current from 220 amperes to440 amperes. In addition, conductive particles flowing through the watercan cause instantaneous shorts capable of destroying the equipment andcausing great damage.

The prior art has recognized these problems and has attempted tocompensate for the changing conductivity of the water by utilizingmechanical devices for moving the spaced-apart electrodes within theheating chamber into a controllable relationship from each other in anattempt to maintain the same resistivity load between the electrodes.The Mamoulides patent mentioned above describes external mechanicalmeans for physically varying the spacing between the spaced-apartelectrodes.

It is envisioned that the operator would increase the spacing betweenthe spaced-apart electrodes in those areas where the water is highlyconductive and conversely reduce the spacing between the electrodeswhere the water has less conductivity. Unfortunately, such prior artdevices cannot adapt to instantaneous changes or other temporary changesin the conductivity or hardness of the water.

In order to minimize the shock hazard associated with electric hot waterheaters, it is necessary that the unit be grounded and that the leakagecurrent between the electrodes and the ground connection be controlledwithin certain limits as determined by the Underwriters Laboratories. Inthe usual installation, the cold water pipes feeding the electric hotwater heater are at ground potential and the water flowing past theelectrodes in the heating chamber determine the leakage path between theelectrodes and the ground. Changing conductivity of the water due toharness or immersed salts or any other reason will also affect theleakage current passing from the electrodes to ground, which current cancreate other dangerous conditions. The prior art has not disclosed howto handle the changing leakage current as a result of the changingconductivity of the fluid medium.

SUMMARY OF THE INVENTION

In the present invention there is disclosed means and apparatus forkeeping the leakage current within limits for extreme conditions of highfluid conductivity. There is disclosed a heating chamber havingspaced-apart electrically conductive electrodes for receiving the liquidto be heated. The electrodes are adapted to be connected to a source ofcontrollable electric power for passing current through the liquidlocated within the heating chamber. The heating chamber has a pluralityof ports, each comprising an enclosure having a predetermined volumedisplacement and configuration which communicate with the heatingchamber at one end through a reduced aperture. In the preferredembodiment there is included an inlet port, an outlet port and a safetyport adapted to be connected to a safety relief valve.

An electrically insulative covering is bonded to and provideselectrically continuous insulation over all surfaces of the heatingchamber and the interior surfaces of the plurality of ports, includingall of their associated enclosures. The leakage path is determined bythe individual connections made to each of the defined paths. Forexample, the inlet port enclosure, including its associated reducedaperture or orifice, connects the input cold water source to the heatingchamber.

The cold water source is usually at ground potential. The leakage pathfrom the electrode to the cold water pipe passes through the volume ofwater within the defined inlet enclosure. The volume and configurationof the enclosure and the aperture communicating the enclosure to theheating chamber is calculated to provide a volume of water having agiven conductivity for limiting the leakage current from the electrodeto the inlet pipe supplying fresh cold water.

Similarly, the output pipe and safety valve are each connected to aground source, thereby establishing leakage paths of limitingconductance through each of the defined ports which include the definedenclosures. The total conductance of all leakage paths is thereforeapproximately limited to the sum of the conductances associated with theleakage paths through each port to ground, which paths are easilydeterminable since the volume and configuration of water within each ofthe enclosures associated with each port is precisely calculable.

In the present invention the electronic controller associated with theheating chamber provides a controllable electric power through theimmersed electrodes in response to the temperature of the water leavingthe heating chamber regardless of variation in conductivity of the watermedium. The electronic controller is connected to the spaced-apartelectrodes and controls the electrical power developed in said liquidbetween said electrodes by limiting the power to a predetermined maximumvalue regardless of changes in water conductivity or external waterdemands made on the system. In addition to the power limiting factor,there is also included a current limiting circuit for preventing currentbeyond a predetermined limit from passing between the immersedelectrodes.

The present invention therefore limits both the power developed in theliquid medium and the total instantaneous current being drawn at anygiven time by the electrodes and is continuously responsive toinstantaneous changes in the output liquid temperature as generated bythe heating chamber.

The novel features which are believed to be characteristic of theinvention, both as to organization and method of operation, togetherwith further objects and advantages thereof, will be better understoodfrom the following description considered in connection with theaccompanying drawings in which several preferred embodiments of theinvention are illustrated by way of example. It is to be expresslyunderstood, however, that the drawings are for the purpose ofillustration and description only and are not intended as a definitionof the limits of the invention.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of the fluid heater and the electronic controllerassembly;

FIG. 2 illustrates the fluid heater and electronic controller assemblywith the covers removed;

FIG. 3 is a sectional view of FIG. 2;

FIG. 4 is a fragmentary view of the fluid heater illustrating theelectric insulative covering;

FIG. 5 is a schematic diagram of the electronic controller; and

FIG. 6 is a series of waveforms illustrating the operation of thecircuit shown in FIG. 5.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1, there is shown a fluid heater 10 and electroniccontroller 12 connected together as a unit and ready for installationwherever instant hot water is required. The electronic controller 12contains a temperature control 14 that is normally used by the operatorto preset the desired output temperature of the water.

A supply pipe 16 of cold water is connected to the lowermost portion ofthe electronic controller 12 which is connected internally with thewater heater 10. In the preferred embodiment a 220-volt 60-hertzsingle-phase service is used to supply the energy for heating the waterand also to operate the electronic controller 12. The source of electricpower is fed through a power opening 18 located in the uppermost portionof the electronic controller 12.

The fluid heater 10 includes an exit port 20 for discharging the hotfluid formed within the fluid heater 10. A safety valve 22 is connectedto a safety port 24 which also communicates with the water heater 10.The safety valve 22 is preset to open should the pressure within thewater heater 10 exceed a given value.

A typical unit of the type illustrated in FIG. 1 for use in aresidential or commercial dwelling will normally use a one-half inchtube or pipe 16 connected to the inlet source and approximately a 3/4inch diameter pipe connected to the output port 20. The inlet watersupply pressure will normally not exceed 100 pounds per square inch as amaximum value, and the safety valve 22 will be preset to open at 125pounds per square inch, thereby insuring that excessive pressures willnot be developed within the water heater 10.

The AC power source may, for example, include a 220-volt service havinga frequency of 60 cycles in a single-phase, three-wire system. Normallythe neutral wire is grounded and the inlet supply as well as the exitpipe from port 20 should be grounded to earth ground in order to preventand minimize any possible shock hazard. Opening any of the hot waterfaucets will automatically supply hot water at the temperature preset bythe temperature control 14.

The normal residential unit will supply hot water at the presettemperature at any flow rate from 0.5 to 4.0 gallons per minute,provided the power required to heat the fluid to the selectedtemperature does not exceed a preselected power level. The embodimentelectronic controller 12 described herein is set to automaticallyprevent excess power beyond a preselected level of 50,000 BTU/hr. frombeing developed in the water heater 10. Exceeding the 50,000 BTU limitwill cause the outlet temperature to decrease as the flow rate isincreased in a manner that will be more fully described in connectionwith the schematic diagram illustrated in FIG. 5.

FIG. 2 shows the fluid heater 10 and electronic controller with thefront covers removed to expose the components. FIG. 3 is a sectionalview of the fluid heater wherein portions of certain components havebeen broken away to show other components which lie behind them.

The cold liquid inlet pipe 16 is connected to a flow switch 30 whichcontains an electrical switch connected to terminals 32 and 34, whichswitch is caused to close whenever fluid is actually flowing through theflow switch 30. A short pipe section 36 connects the flow switch 30 withan inlet port enclosure 44 of the heating chamber 54. A pair of siliconcontrolled rectifiers (SCR's) 38 and 40 are cast into a block of epoxy46 surrounding pipe section 36. The epoxy is of a type, known to theart, which is thermally conductive and electrically insulating. Theblock serves as a heat sink for the SCR's. The heat generated by theSCR's preheats the liquid thereby increasing the efficiency of theheater.

The pipe section 36 fits within the inlet port enclosure 44. An inletport 43 leads to a passage 48 that is closed at the opposite end by aplug 50 and that communicates with heating chamber 54 through passage52.

Exit port 20 is located at the uppermost end of the heating chamber 54.Exit port 20 has an enclosure 56 into which an output pipe 58 isattached.

Safety port 24 has an enclosure 60 to which a safety valve 22, whichincludes a short threaded section, is attached. Excess pressuredeveloped within the heating chamber 54 will cause the safety valve 22to open, thereby relieving the excess pressure.

A pair of plate-like electrodes 62 and 64 face each other within theheating chamber 54. The electrodes are mechanically held in place andseparated from each other by suitable spacers (not shown).

Copper straps 68 and 66 are held in electrical contact with thenon-opposing outer surfaces of the electrodes 62 and 64.

In FIG. 2 the front wall or cover of the enclosure of the heatingchamber 54 and the suitable spacers have been removed to show the partsinside. In the sectional view of FIG. 3, the inner or opposing face ofthe rear electrode 64 is visible behind the outer surface of the nearerelectrode 62 (shown as broken away in FIG. 3).

The source of electrical power from 18 is connected from terminals 74 tostand-off insulators 70 and 72 through the SCR's 38 and 40 and through acurrent sensing transformer 76 back to the terminals 74. The electroniccontrol circuits for controlling the firing times of the SCR's 38 and 40are contained on a panel board 78. Details of the individual connectionsfor controlling the firing of the SCR's and for limiting the power to amaximum of 40,000 BTU/hr. will be more fully described in connectionwith the description of the schematic diagram illustrated in FIG. 5.

Referring now to FIG. 4, there is shown a fragmentary view of FIG. 3which more fully illustrates the electrically insulative covering 80which covers all interior surfaces and in the preferred embodimentcompletely covers the internal and external surfaces of the water heater10. In the preferred embodiment, an epoxy coating is placed over allsurface portions of the water heater 10, including the inlet enclosure42, the inlet port 44, the channel 48, the plug 50, the central opening52, the heating chamber 54, the exit port 20, the exit enclosures 56,the safety port 24 and the safety enclosure 60. In other words, allportions of the water heater 10, both inside and outside, are coveredwith the electrically insulative coating 80 which is made contiguouswith and electrically continuous over the surfaces of the water heater.

The individual enclosures associated with each of the ports, such asinlet enclosure 42, exit enclosure 56 and safety enclosure 60, togetherwith the electrically insulative coating 80, are all used as a means ofcontrolling the leakage current from the individual electrodes 62 and 64to earth ground.

The purpose of the electrically insulative coating 80 is to prevent anyleakage current flowing directly from the electrodes 62 and 64 to anyportion of the casing associated with the water heater 10. It should beremembered that the water heater 10 may be constructed of a conductivemetal, such as aluminum. By coating all surfaces of the water heater 10with the electrically insulative coating 80, any leakage current fromthe electrodes 62 and 64 must flow from the electrodes through the waterconfined in the port enclosures to either the input pipe source 16 whichis at earth ground or to the exit pipe which is also at earth ground.

Those skilled in the art will quickly recognize that the safety valve22, when locked in place within the safety enclosure 60 by means of pipethreads associated with the aforementioned parts, will invariably cutthrough the coating 80, thereby causing the safety valve 22 tophysically contact the metal casing forming the water heater 10. Inother words, the physical presence of the safety valve 22 will thereforepresent an additional leakage path from the individual electrodes 62 and64 through the water which is in contact with the safety valve 22 backthrough the threads contacting the safety valve and to the metal casingof the water heater 10.

There is still a possibility of a fourth leakage path to ground whichincludes the plug 50 inserted into one end of the channel 48. Normallythe plug 50 is also coated with the insulative coating 80 so thatinserting and removing the plug 50 will not cause the water locatedwithin the chamber 48 to touch or be in contact with any exposed metalportion of the plug 50. However, it is envisioned that through use, theopening and closing of plug 50 may remove portions of the coating 80thereby allowing water or fluid within the channel 48 to establish afourth leakage path from the electrodes 62 and 64 through the water andthrough the exposed metal on the plug 50 back through the metal portionsof the water heater 10 to earth ground. It will be recognized of coursethat plug 50 and its associated portions of channel 48 is not necessary,and may readily be eliminated in other embodiments of the invention.

In an electrode type water heater it is well known that when any of theelectrodes are connected to a power supply that the leakage current willexist from the electrode to some point of common ground in view of thegeneral practice of grounding the power supply to minimize potentialshock hazards. These leakage currents exist because of the specificconductance of water which is usually measured and expressed in terms ofmicro-mhos per cubic centimeter.

In the present invention, the use of an insulated covering 80 over allportions of the water heater 10 causes the leakage current to bedirected through the water confined by the enclosures in four givenlocations, namely the inlet portion, the exit portion, the safety valveportion and the plug portion. A control or limit of the magnitude of theleakage current is possible since the voltage of the power supply isknown and the specific conductance of the fluid involved is determinedfor a worst case situation which has been determined to be potable waterhaving a conductance of 1660 micro-mhos per cubic centimeter. The onlyunknown is the volume of water located between the electrodes and earthground.

The individual enclosures such as the inlet enclosure 42 and the exitenclosure 56 together with the safety enclosure 60 includingparticularly the corresponding restricted apertures or orifices areconstructed to hold predetermined volumes and geometric configurationsof liquid. In general, the electrical resistance presented by anyportion of a water volume to passage of current varies directly withlength and inversely with area. Thus, it is evident that those portionsof the individual enclosures which constitutes the narrow diameterextended restricted apertures or orifices presents relatively highresistance to the passage of leakage current and will be the primarydeterminant in limiting the leakage current to predetermined levels.

In the practical embodiment involving the use of electric water heaters,the Underwriters Laboratories have determined a maximum value forleakage current and that maximum value is used to predetermine the sizeand configuration of the individual enclosures and restrictive orificesassociated with each of the ports feeding the heating chamber. In otherwords, for a given allowed leakage current, a given size enclosure isconstructed to contain a given volume and configuration of liquid whosetotal conductance will be sufficiently low to so limit the leakagecurrent from the electrodes to the common ground connection.

The end plug 50 does represent a potential fourth leakage path to acommon ground, however, the resistance of the water path contained inthe channel 48 is substantially greater than that in any of the otherleakage paths, and since all leakage paths are in parallel, it can beeasily determined that the addition of a fourth, high-resistance path,in parallel with three relatively low resistive paths, will not decreasethe total resistivity of the overall system to any significant extent.

Referring now to FIG. 5, there is shown a schematic diagram illustratingthe electronic components for controlling the flow of electrical powerinto the water to be heated. The 220-volt input line is fed to the fieldwiring terminals 74 which connect an optional radio frequencyinterference choke 82, the current sensing transformer 76 and the SCR's38 and 40 in a circuit with the immersed electrodes 62 and 64. Thedefined circuit is normally open until either of the SCR's 38 and 40 arerendered conductive so as to allow current to pass between theelectrodes 62 and 64 for the period of time that the SCR's conduct.

The individual SCR's 38 and 40 are controlled by a transformer 84 havinga single primary winding 86 and a pair of identical secondary windings88 and 90. With the primary windings 86 energized, identical voltagesbut of opposite polarity will be induced in secondary windings 88 and 90which are connected to SCR's 38 and 40 respectively. For the first halfcycle, SCR 38 will be energized and current will flow between theelectrodes 62 and 64. For the second half cycle, SCR 40 will beenergized and SCR 38 extinguished, thereby allowing current to flowbetween electrode 62 and 64.

It will be recognized that, for any complete cycle of the 60-cycle waveimpressed on the primary windings 86, the individual SCR's will each beturned on for a selected portion of the one-half cycle. The process willrepeat itself for each half cycle at which point the second SCR will beturned on and the first SCR will be extinguished as the impressed signalpasses through zero. For any given sine wave, the positive going halfcycle and the negative going half cycle will be substantially identical,thereby insuring that SCR 38 and 40 will be conducting for similarlengths of time thereby insuring a balanced current flow over a singlecycle.

Control over the amount of heat generated in the fluid flowing betweenelectrodes 62 and 64 is determined by the firing angle or time in eachcycle that the individual SCR's are turned "on" and rendered conductive.In other words, with a small demand for heat, conduction of the SCR'swill take place "late" in the cycle resulting in a short firing anglefor each SCR. Conversely, a heavy demand for hot water will result inthe individual SCR's firing "early" in the cycle, thereby insuring thatthe individual SCR's are on for a maximum time or large firing angle.The individual SCR's 38 and 40 are wired back to back, thereby allowingeach SCR to be controlled for a given half cycle while the other SCR isextinguished.

Operating potential for the unit is achieved from the line voltage beingfed to a full wave rectifier 94 having a zener diode 96 in the outputfor limiting the output voltage on the output line 98.

The primary winding 86 of transformer 84 is connected to the output line98 through a diode 100, and through a through a transistor 102, throughthe flow switch 30 to ground. The transistor 102 is normallynonconductive with the base connected to one terminal of a normallynonconducting unijunction transistor 104. The unijunction transistor 104and associated components form a pulse generator having a controllablefrequency depending upon the heat requirements of the external circuit.

A high frequency oscillation of the pulse generator will causeunijunction 104 to fire early in the half cycle causing transistor 102to fire early, whereas a lower frequency of oscillation of the pulsegenerator transistor 104 will cause the unijunction transistor 104 tofire late in the half cycle, resulting in transistor 102 firing late inthe cycle. In the preferred embodiment, the pulse generator varies overa range including 1000 cycles per second.

The temperature of the water is measured at the exit port by means of athermistor temperature sensor 106 which feeds the input line 108 or basecircuit of a transistor 110 that is connected as a differentialamplifier with transistor 12.

The base circuit of transistor 112 is connected across a droppingresistor and to a variable calibrating resistor to ground. The basecircuit of transistor 110 on the other hand contains a resistive circuitincluding variable resistor 14 identified as the temperature control andthe thermistor 106 to ground. During a balancing operation, the basecircuits of the differential amplifiers are calibrated and balanced. Avariation in the thermistor 106, indicating a need for more heat, willcause the input line 108 to the base of transistor 110 to vary from thebalanced condition and cause transistor 110 to conduct less and forcetransistor 112 to conduct more.

In the normal condition, transistor 112 will be conducting during eachpart of the half cycle that heat is required, thereby charging capacitor114 until the voltage on the capacitor reaches the breakdown potentialof approximately 4.8 volts required for the unijunction 104 to fire. Thevarying impedance of transistor 112 causing the charging of capacitor114 will therefore determine the time in the cycle when the unijunctiontransistor 104 fires. The rate of charging of capacitor 114 willdetermine the time in the cycle when transistor 102 fires.

With transistor 112 having a high impedance, capacitor 114 will chargeat a relatively slow rate and the pulse rate will be substantially lowand hence transistor 102 will fire late in the half cycle.

A review of the circuit will show that the individual SCR's 38 and 40are each coupled through transformer 84 to be alternately turned on bythe successive firings each half cycle of transistor 102 andindividually turned off as the 60-cycle wave passes through zero. Thefiring angle or the time that the SCR's are turned on is a function ofwhen in the corresponding half cycle transistor 102 is impulsed by theunijunction transistor 104.

The charging of capacitor 114 past the critical voltage of theunijunction transistor 104 will render the unijunction conductive andhence fire transistor 102; however, the rate of charge of capacitor 114is determined by the impedance of transistor 112 comprising one-half ofthe differential amplifier. In other words, the rate of charge of thecapacitor 114 is a function of the variable impedance of the transistor112 and the size of the capacitor 114 and will vary exponentially inaccordance with an RC charging circuit.

Transistor 112 is rendered conductive each half cycle of the 60-cycleinput line and for each half cycle charges capacitor 114 at anexponential rate. The time required for for capacitor 114 to reach thethreshold voltage of the unijunction transistor 104 is therefore afunction of the impedance of transistor 112.

Hence it can be appreciated that whenever a small amount of heat isrequired transistor 112 will be conducting for a short period of time ineach half cycle, and hence capacitor 114 will charge to the thresholdvoltage over a longer period of time as determined by the RC timeconstant. The longer it takes capacitor 114 to charge to the thresholdvoltage of the unijunction transistor 104, the smaller the firing angle,since the SCR's 38 and 40 will be turned on for a very short period oftime before the 60-cycle wave passes through zero and extinguishes theconduction. The less time in the cycle that the SCR's 38 and 40 areconducting, the less power will flow through the electrodes 62 and 64,and hence the less heat will be generated in the fluid medium.

Consider now a situation where the voltage balance between temperaturesensor 106 and resistor temperature control 14 indicates a lowtemperature condition requiring the generation of heat. In such asituation, the unbalance between the temperature sensor 106 and theresistor temperature control 14 in the base circuit of transistor 110will present a voltage across line 108 and ground which causes thetransistor 110 to be less conductive and thus allows transistor 112 toconduct more heavily, thereby presenting a lower impedance in thecharging circuit of capacitor 114. The rate of charging of capacitor 114is therefore increased allowing the capacitor 114 to reach the thresholdpotential of the unijunction transistor 104 at an earlier time whichcauses the firing of transistor 102 and results in the SCR's 38 and 40firing for a longer period of time during each half cycle.

Capacitor 114 will discharge through the unijunction transistor 104 toground until extinguished. Transistor 112 will attempt to again chargecapacitor 114, which process will continue for the one-half cycle untilzero voltage stops all conduction. During the one-half cycle anddepending on how "hard" transistor 112 is driven into conduction, it ispossible for capacitor 114 to charge and discharge through theunijunction transistor 104 at a varying rate of up to 1000 pulses persecond.

At the higher pulse rate, which represents a high charging rate for thecapacitor 114, the SCR's 38 and 40 are on for a longer period of time ofthe total half cycle. A lower pulse rate is achieved when transistor 112is conducting at a lower rate, indicating a higher impedance, and hencethe charging rate of the capacitor 114 takes a longer period of time.The number of discharges of capacitor 114 through the unijunctiontransistor 104 is less, thereby allowing the SCR's 38 and 40 to be onfor a shorter period of time for each half cycle.

Under normal conditions, the base currents of the differentialamplifiers 110 and 112 will be substantially equal and the transistors110 and 112 will conduct "on" and "off" depending upon the slightvariations as the base currents vary. A drop in the temperature of theliquid, as determined by the temperature sensor 106 and the pre-settemperature control 14, has the effect of unbalancing the base currentscausing transistor 110 to conduct less and transistor 112 to conductmore.

Referring now to FIG. 6, there is shown a series of waveforms more fullyillustrating how the firing angle is changed in response to the rate ofcharging of capacitor 114. Curve 120 illustrates a single cycle of the60-cycle input appearing at the terminals 74. Curve 122 illustrates asubstantially full wave rectified signal of the sine wave 120 whichappears at the output of the full wave bridge rectifier 94 on outputline 98.

In actual practice, the curve 122 is clipped and maintained at asubstantial constant output level for each 180° due to the action of thezener diode 96. The changing impedance of transistor 112 changes thecharging rate of capacitor 114, and as previously described, thecharging rate of the capacitor is a function of the interaction oftransistors 110 and 112 which are connected as differential amplifiers.

The firing potential of the unijunction transistor 104 in the preferredembodiment is 4.8 volts, which means that capacitor 114 will charge to4.8 volts and then discharge to the threshold of the unijunctiontransistor 104, which action will occur for a plurality of pulses forthe first 180°. Curve 124 illustrates a substantially high charging rateof capacitor 114, resulting from a heavy demand for current as evidencedby a low impedance driving source which causes a substantially fastcharge rate for capacitor 114 through unijunction transistor 104.

Upon the very discharge of the unijunction transistor 104, transistor102 is fired and, depending upon the polarity of the secondary windings88 and 90 of transformer 84, the SCR 38 will fire at time T-1 asillustrated in Curve 125. SCR 38 will therefore be on for a long firingangle measured from T-1 to 180° at which point the voltage envelopepassing through zero will extinguish the firing of the SCR.

The continuing charging and discharging of capacitor 114 for the periodof time from 180° to 360° is shown by Curve 128, which is basically arepetition of Curve 124. Upon the Curve 128 reaching the thresholdpotential of 4.8 volts, the unijunction transistor 104 is fired at timeT-2, as illustrated in Curve 129, thereby allowing SCR 40 to fire for atime measured from T-2 to 360°, which is substantially the same time asfrom T-1 to 180°.

In a similar manner, Curve 130 illustrates a slower charging rate forcapacitor 114 for the first half cycle whereas Curve 132 illustrates asimilar charging rate for the second one-half cycle. The firing anglefor the SCR 38 as shown for the charging rate of Curve 130 is shown onCurve 131 as being measured by the time between T-3 and 180° for thefiring angle of SCR 38. The firing angle for the SCR 40 is shown onCurve 133 as the firing angle between T-4 and 360°.

The heating of flowing water presents additional problems which resultsfrom the fact that minerals, salts and metal particles can exist in thewater in spite of purfication techniques presently being used today.Conductive particles lodge between electrodes 62 and 64 present thepossibility of generating short circuit currents when the SCR's 38 and40 are rendered conductive.

These short circuit currents can generate hundreds of amperes for eachhalf cycle that the SCR's conduct and have the potential possibilitiesof destroying the equipment if these currents are not controlled. Ashort circuit protection scheme is disclosed which detects a high pulseof current in excess of a predetermined amount, which current pulse iscaused to control the potential across the base of transistor 110 so asto render transistor 110 conductive, thereby cutting off transistor 112by presenting a high impedance and preventing capacitor 114 from beingcharged. With the capacitor 114 not charging, it is then impossible foreither of the SCR's 38 or 40 to fire.

As shown in FIG. 6, a current sensing transformer 76 is located incircuit with the power lines feeding electrodes 62 and 64. Individualpulses of current will be detected by the transformer 76 and fed to afull wave bridge rectifier 140 which generates a DC potential output online 142 which has a voltage amplitude bearing a direct relationship tothe amplitude pulse of current detected by transformer 76. Line 142feeds a voltage divider consisting of resistors 114 and 146 feeding atransistor 148 connected in circuit as a diode.

In the preferred embodiment, it was arbitrarily determined that a pulseof current in excess of approximately 300 amperes for one-half cyclewould be the cutoff point for stopping the conduction of SCR's 38 and40. The voltage divider of resistors 144 and 146 was selected togenerate an amplitude of voltage sufficient to cause transistor 148 toconduct when the detected current was in excess of 300 amperes. Firingof transistor 148 causes a capacitor 150 to charge through diode 152 forthe remainder of the one-half cycle. Capacitor 150 is also connected tothe base of a transistor 154 and is prevented from discharging throughtransistor 148 by means of the back biased diode 152. The emitter oftransistor 154 is connected to ground through a voltage divider circuitconsisting of resistors 156 and 158 which are connected from ground tothe output line 98.

The voltage divider of resistors 156 and 158 determine the thresholdvoltage needed to fire transistor 154. Whenever the input voltageappearing across the base of transistor 154 is greater than thethreshold voltage, the transistor will fire and thereby place a lowimpedance path from the input line 108, which is the base connection fortransistor 110 to ground. This action will immediately cause transistor110 to conduct heavily, thereby effectively cutting off transistor 112which prevents capacitor 114 from being charged.

The value of capacitor 150 is selected in combination with resistor 158so that the discharge path for the capacitor 150 through the transistor154 and through resistor 158 to ground will take approximately 30seconds to maintain transistor 154 conducting. Transistor 110 willinitially conduct heavily, thereby cutting off transistor 112 whichprevents capacitor 114 from charging. Recognizing that the discharge ofcapacitor 150 is exponential, it will be recognized that the chargingvoltage on the base of transistor 154 will change the impedance oftransistor 154 from an initially low value to a gradually higher valueuntil the transistor is cut off. The increasing impedance of transistor154 gradually restores the base of transistor 110 to the normal controlof the temperature sensor 106.

Once transistor 154 stops conducting, the input line line 108 is nolonger clamped to a low impedance source and the normal voltage dividercurrent flowing through resistors 156 and 158 will increase, allowingthe voltage across line 108 to ground to slowly allow transistor 112 toconduct to begin the charging of capacitor 114 and the ultimatereturning of electrical power to the water heater by the conduction ofSCR's 38 and 40.

Toward the end of the 30-second delay time it will be appreciated thattransistor 112 will come on very slowly due to the normal differentialaction of transistors 110 and 112. The charging rate of capacitor 114will therefore be taking place at a slow rate due to the initial highimpedance of transistor 112, as shown in FIG. 6 and as measured by timeT-5 to 180° and time T-6 to 360°. Should the shorting conditions stillexist between the electrodes 62 and 64, the small firing angle for bothSCR's 38 and 40 will prevent excessive current from passing through theline until the short is cleared.

In other words, the short circuit protecting scheme just described willinitially detect the high surge of current caused by the short circuitand immediately result in a 30-second shutdown of power to the SCR's.The returning power to the SCR's will take place initially over theshort firing angle until the short circuit conditions are removed. Afterthe initial surge of short circuit current, the normal action ofcontrolling the charging rate of capacitor 114 will initially limit thecurrent fed to the SCR's to a minimum value.

The circuit therefore is self-correcting in that the removal of theshort circuit conditions allows the slow application of full power untilnormal conditions are achieved. Should the short circuit conditionscontinue to exist, the application of initial power is kept to a minimumand the current developed by the SCR's is kept to a minimum bymaintaining a very short firing angle for each SCR until the conditionis removed.

There is included a power limiter which effectively limits the powergenerated between electrodes 62 and 64 to a predetermined value in viewof the size of the water chamber, the flow rate desired, and theavailable common electric service. In the preferred embodiment, a valueof 50,000 BTU/hr. was selected as the maximum power that the unit wouldbe allowed to impart into the water during the heating process. (This isapproximately the amount of power that would be applied by a steadystate current of 65 amperes at 225 volts.)

The BTU or power limiter to be described actually limits the amount ofpower that the system is allowed to generate in the water being heatedand is effective regardless of changes in conductivity of the watermedium or in instantaneous values or duration of the pulsed chargingcurrent.

The power limiter utilized the output voltage from the rectifier 140 asdetected by the current sensing transformer 76. The output line 142 isconnected through a diode 160 and a limiting resistor 162 to a capacitor164. The charging rate of the capacitor 164 is accurately determined inresponse to the actual firing angles or time that the SCR's 38 and 40are conducting. A transistor 166 is normally conducting and is connectedacross the capacitor 164, thereby allowing the capacitor to continuouslydischarge through transistor 166. The base of transistor 166 isconnected to the collector terminal of a transistor 168, which is alsoconnected across the capacitor 164. Transistor 168 is normally held in anonconductive condition and has a base that is connected intermediate avoltage divider comprising resistors 170 and 172 that are connectedbetween the operating potential on line 142 and ground.

Since transistor 166 is normally conducting, the capacitor 164 isincapable of being recharged until the transistor is held in anonconducting condition by the action of transistor 168. The purpose ofthe circuit is to allow the capacitor 164 to charge only when the SCR's38 and 40 are conducting and at the same time allow the charging voltageon the capacitor 164 to increase in response to an increase in theamplitude of current flowing through the SCR's 38 and 40. In otherwords, the charging of capacitor 164 will be a function not only of theamplitude of current flowing through the SCR's but also for the lengthof time that the SCR's are conducting, thereby making the ultimatecharge on capacitor 164 representative of the actual power passed by theconducting SCR's 38 and 40.

The flow of current through the conducting SCR's 38 and 40 will bedetected at the current sensing transformer 76 and presented as avoltage on output line 142 from the full wave rectifier 140. A voltageappearing across line 142 and ground appears across the voltage dividerof resistors 170 and 172, thereby producing a voltage across the base oftransistor 168 causing the transistor to conduct. The conduction oftransistor 168 immediately causes transistor 166 to stop conducting,thereby allowing capacitor 164 to charge through diode 160 for the fullfiring angle that the individual SCR's 38 and 40 are conducting. Themagnitude of the voltage appearing on line 142 will determine thecharging potential. The time of charging will be determined by thelength of the firing angle. At the end of each half cycle and for theoff time, transistor 166 conducts and allows the capacitor 164 todischarge at a controlled rate as determined by resistors 174 located inthe emitter path of transistor 166. The process then continues for eachhalf cycle with transistor 166 conducting during the off time anddischarging capacitor 164 and transistor 166 held nonconducting by theaction of transistor 168 during the firing angle time of the SCR's 38and 40, which allows capacitor 164 to charge.

The output of the capacitor 164 is fed to a decoupling networkcomprising resistor 176 and capacitor 178, which is connected to thebase of a transistor 180. The emitter of 180 is connected across thevoltage divider consisting of resistors 156 and 158 previously describedin connection with the short circuit protection device. As described inconnection with the operation of transistor 154, the threshold voltageacross the voltage divider 158 is selected to keep transistor 180 in anormally nonconducting state. The circuit parameters are chosen so thatthe voltage appearing across capacitor 164 is representative of thepower generated between electrodes 62 and 64, as evidenced by the valueof current passing through the SCR's 38 and 40 and the time of firingangle that each of the SCR's are conducting. The value of voltageappearing across capacitor 164 is selected to be representative of thevalue 50,000 BTU's/hr., and when this predetermined value is reached,the base of transistor 180 will cause the transistor to begin toconduct, thereby establishing a shunt path line 108 (which is the baseof differential amplifier transistor 110) through transistor 180 andresistor 158 to ground. This connection shifts the operating point oftransistor 110 to produce a shorter firing angle and thereby limit thepower to the 50,000 BTU/hr. level.

The filtering action of resistors 176 and capacitor 178 on the base ofthe transistor 180 maintains the transistor 180 in a conducting statefor a period of time as determined by the voltage appearing across theinput capacitor 164. As the power generated by the SCR's 38 and 40decreases, the charge on the capacitor 164 will slowly dissipate throughtransistor 166 and resistor 174, thereby allowing the base voltageappearing on transistor 180 to slowly decrease. This action slowlychanges the impedance of transistor 180 and gradually allows the inputline 108 feeding the base of transistor 110 to approach a normaloperating potential. This normalizing of the input voltage on the baseof transistor 110 slowly allows transistor 112 to become conducting,which places a substantially high impedance in a charging circuit ofcapacitor 114 which slowly again allows the SCR's 38 and 40 to becomeconducting.

Currents in excess of 300 amperes will be detected by the short circuitprotection circuit as previously described; however, these largecurrents will not unduly affect the power limiter circuit unless thefiring angle is large and the total power is in excess of 50,000BTU's/hr., as represented by the charging voltage on the capacitor 164.

A review of the circuit will show that the short circuit currentprotection system is independent of the power limiter and converselythat the power limiter is independent of the instantaneous currentflowing at any one time since power is basically a function of thecurrent flowing and the length of time that the current flows.

It is clear that the present invention is not limited to water, but canbe used to heat any fluid that can be made to be electricallyconductive.

Other embodiments of the present invention and modifications of theembodiments presented herein may be developed without departing from theessential characteristics thereof. Accordingly, the invention should belimited only by the scope of the claims appended below.

What is claimed as new is:
 1. For use with an electric fluid heaterhaving spaced-apart electrodes for applying electrical current to afluid, and having a temperature sensor for sensing the temperature ofthe fluid, an electronic temperature control system comprising:(a) powercontrol means connectable to the electrodes and operable for applyingpower to the electrodes, said power control means being responsive toapplication of a control signal thereto for controlling the electricpower output in accordance with an electrical characteristic of thecontrol signal; and, (b) control circuit means connected to said powercontrol means and adapted to be connected to the temperature sensor,said control circuit means normally generating the control signal, saidcontrol circuit means being responsive to signals from the temperaturesensor for altering the electrical characteristics of the control signalwhenever the temperature of the fluid varies from a preset value, saidcontrol circuit means including means for sensing the electric currentsupplied to the electrodes, and further including a power limitingcircuit, responsive to the sensed electric current for output generatinga power detection signal whose magnitude represents the power beingapplied to the electrodes, said control circuit means being responsiveto said power detection signal exceeding a preset value for altering thecontrol signal to decrease the power applied to the electrodes.
 2. Thesystem defined by claim 1 in which said power control means controls theelectric power applied to the electrodes by alternately blocking andpassing current to the electrodes, said power control means controllingthe relative durations of blockage and passage of current in accordancewith the electric characteristic of the control signal.
 3. The systemdefined by claim 1 in which said power limiting circuit produces acurrent signal representative of the current supplied to the electrodesand quasiintegrates the current signal to generate the power detectionsignal.
 4. The system defined by claim 1 in which said control circuitmeans further comprises:a peak current circuit responsive to the sensedcurrent for producing an inhibit signal of substantial duration wheneverthe current supplied to the electrodes exceeds a predetermined value,said control circuit means being responsive to the inhibit signal forinhibiting generation of the control signal during the duration of theinhibit signal.
 5. The system defined by claim 1 in which said controlcircuit means further comprise:(a) a capacitor; (b) means for cyclicallycharging said capacitor at a variable charging rate representative ofdeviation of the temperature of the fluid from the preset value; and (c)means responsive in each charging cycle to the charge in said capacitorfor generating the control signal.
 6. The system defined by claim 5 inwhich said control circuit means is responsive to the power detectionsignal for reducing the charging rate of said capacitor.
 7. The systemdefined by claim 5 in which said control circuit means further comprisesmeans responsive to the sensed current for inhibiting charging of saidcapacitor for a substantial period of time whenever the current suppliedto the electrodes exceeds a predetermined value.